Non-powered air-purifying respirator equipment involves a breathing mask having a filtered air inlet. Air is drawn through the filter by means of the wearer's breathing action. A considerable problem with this type of respirator is how to determine when the filter is due to be replaced. A number of "end-of-service-life" indicators have been proposed over the years, but none have been widely adopted. The major difficulty is that the useful life of the filter is determined by several non-related factors, such as the proportion of contaminant in the atmosphere, the humidity and the effort required of the user. Present estimates of filter lifetime are based on a number of such factors, and it takes considerable experience to weigh them together.
In recent years positive air-pressure respirators have been introduced, and these employ a pump which draws ambient air in through a filter and supply it to the face mask. The pump comprises a motorized fan which draws air through the filter in proportion to the speed of revolution. In such simple motorized equipment the filter life, in a particular environment, is directly related to the operating time and in practice can be estimated with reasonable reliability. However, these respirators suffer from the problems that they do not necessarily provide sufficient air flow for periods of maximum inhalation, but are otherwise wasteful in filter usage by providing excess flow during exhalation cycles.
A new generation of powered air-purifying respirators (PAPRs) that has been developed by the applicant employs a breathing demand valve to overcome the deficiencies of the simple positive air-pressure respirators mentioned above. However, the inclusion of the demand valve has reintroduced the unpredictable variant of air consumption into the determination of filter life.